Intravascular stents are well known in the field of interventional cardiology for the treatment of arterial stenoses. When placed through the body's vascular system, most stents are mounted onto a balloon angioplasty catheter with or without a cylindrical sheath that covers the stent prior to stent deployment by balloon expansion at the site of a dilated stenosis. If a sheath is not used, the stent can get caught on a calcified plaque or on the guiding catheter during movement into or out of the patient's body. This can cause the stent to come off the delivery catheter and embolize into the vasculature. When a sheath is used, it can have a few disadvantages. A first disadvantage is that most prior art sheaths are secured only at the stent delivery catheter's proximal end. Therefore, in order to have sufficient column strength, the sheath must be relatively thick-walled, making it stiff and bulky so that passage through tortuous coronary arteries can be difficult. Another disadvantage of prior art sheaths is that they have blunt distal ends that can catch on an already deployed stent, or on a calcified piece of intimal dissected tissue or on a tight stenosis. Still further, when secured only at the proximal end of a stent delivery catheter, the sheath often either uncovers the stent due to significant bending of the stent delivery catheter or the sheath advances too far distally beyond the distal end of the stent.
Because of the comparatively larger diameter, blunt end and stiffness of extant sheathed stent delivery systems or because of the rough outer surface of unsheathed stent delivery systems, pre-dilatation with another balloon angioplasty catheter is almost always required before stent implantation. It is highly desirable to be able to deliver a stent without pre-dilatation. The procedure of deploying a vascular stent without pre-dilatation and post-dilatation is called "primary stenting". In U.S. Pat. No. 5,792,144, Fischell et al describe a primary stenting system having a membrane sheath over the distal section of the catheter which holds the stent. While the Fischell device provides the capability for primary stenting, it may not hold the distal end of the membrane sheath with sufficient firmness to prevent it from uncovering the stent during passage through curved or calcified vessels. In addition, the membrane sheath of the Fischell et al invention adds several thousandths of an inch to the stent system profile. It would be advantageous if the stent delivery systems' outside diameter could be made smaller by shrinking the membrane sheath down onto the stent or, alternatively, having a primary stenting system without a membrane sheath.